Ampeg BA115 Blows Fuse

[misterc57]

Previous owner stated it blows fuse.

I connected to my variac. A slight turn from 0 V moves the current needle quickly towards 1 A.

I am always stuck on where to start on these high current issues on SS amps.

Q3 was encased in white thermal grease, I removed the grease to see what was there.

Advice greatly welcomed.

Schematic is attached.


Ampeg_BA115A_Schematic.pdf

[Jon Snell]

That thermal grease is to ensure good thermal tracking for the quiescent current in the output stage.
Put it back.
Either Q2 / Q5 are short circuit or the mains transformer has issues ... maybe a faulty diode in the bridge rectifier of the power supply.
This assumes the mains supply is the correct voltage.

[misterc57]

I cannot make out the print on the transistors Q2 and Q5.

Assuming the pins left to right are E C B, then on each, B is shorted to both E and C

[g1]

They are Fet's, not bipolar transistors. So no EBC, but gate, source, and drain.
Schematic attached.

[DrGonz78]

Yeah I still have a batch of 9140r & 140r. I got a bunch a while back thinking I might use them but nope. Not sure if they are easily sourced these days. Don’t neglect to replace those mje340/350 pre-drivers if you have to replace the FETS.

[misterc57]

Found a layout and parts list, attached.

So... replace

Q4 96-034-01 MJE340 NPN 300V 20W .5A 72 73 1
Q1 96-035-01 MJE350 PNP -300V 20W -.5A 73 74 1

Q2 96-914-01 IRFP9140R P-CH -100V 150W -19A 74 75 1
Q5 96-014-01 IRFP140R N-CH 100V 125W 27

what about Q3?

Q3 96R006-01 MPSA06 NPN 80V 1.5W .5A 75 76 1


103XXB_1[1].pdf

[misterc57]

Any tips on how to remove the output transistor? I unsoldered and removed the screw. It will not left off the heatsink.

Thank you

[g1]

Those are sil pads sticking the Fets to the heatsink. You will need to replace them as well. Grip the fets by the sides with pliers and give them a twist, they should break free.

[Jon Snell]

I use a work knife and place the sharp blade flat detween the aluminium and the insulator. With a gentle tap with a small implement on the blade, it will release easily. Sometimes it is re usable but best to replace them.
They are Power HexFets, not BJTs.